Why Is Glass Transparent?

or brick that surrounds it. After all, both materials are solid, and both keep out rain, snow, and wind. Yet wood is opaque and blocks like completely, while glass is transparent and let's sunshine stream through unimpeded. You may have heard some people and even some science textbooks try to explain this by saying that wood is a true solid and that glass is highly viscous liquid. They go on to argue that the atoms in glass are spread farther apart

and that these gaps let like squeeze through. They may even point to the windows of centuries old houses, which often look wavy and unevenly thick, as evidence that the windows have flowed over the years, like the slow crawl of molasses on a cold day. In reality, glass isn't a liquid at all. It's a special kind of solid

known as an amorphous solid. This is a state of matter in which the atoms and molecules are locked into place, but instead of forming neat orderly crystals, they arrange themselves randomly. As a result, glass is mechanically rigid like a solid, yet it has the disordered arrangement of molecules like liquids.

Amorphous solids form when a solid substance like silicon dioxide also known as silica or sand, is melted at high temperatures and then cooled so fast that it doesn't have time to form orderly crystal, a process known as quenching. The panes of glass in old houses aren't wavy and thicker at the bottom because they're still flowing. They were made in a time when glass technology wasn't as good as it is today, so each pain may have had some rippling in it when it's set and been thicker

on one end than the other. The carpenter would have put the thicker end on the bottom because it's sturdier like that. In many ways, glasses are like ceramics and have all of the same properties durability, strength and brittleness, high electrical and thermal resistance, and a lack of chemical reactivity. Basically, glass won't corrode, break down, or discolor, which is why

it's used in so many applications. What's called soda lime glass, which is the commercial glass that you find in sheet and plate glass glass, jars and bottles and light bulbs, has another important property. It's transparent to the range of electromagnetic wavelengths known as visible light. To understand why, we have to take a closer look at the atomic structure of glass and understand what happens when photons, the smallest

particles of light, interact with that structure. Okay, so soda lime glass is made up of mostly silicon dioxide or silica, with a little bit of sodium carbonate or soda ash added for manageability, and calcium carbonate or lime added for hardness and durability. All of those atoms are arranged in the amorphous solid body of the glass. Now, think about the structure of an atom. You've got the atom's nucleus with any protons and neutrons it has, and then the

electrons around that occupying different energy levels. If an electron gains energy, it might move to a higher energy level. If it gives up energy, it might move to a lower one. In either case, the electron can only gain or release energy in discrete bundles. Light is made up of photons. Now, let's that are a photon moving toward and interacting with a solid object made up of atoms. One of three things can happen. First scenario, the substance

could absorb the photon. This occurs when the photon gives up its energy to an electron located in the material. Armed with this extra energy, the electron is able to skip to a higher energy level while the photon disappears. Second scenario, the substance could reflect the photon. To do this, the photon gives up its energy to the material, but a photon of identical energy is emitted. Third scenario, the substance could allow the photon to pass through unchanged, known

as transmission. This happens because the photon doesn't interact with any electrons and continues its journey until it interacts with another object. Clear glass falls into this last category. Photons pass through the material because they don't have sufficient energy to excite a lo electrons in the glass to a higher energy level. Physicists sometimes talk about this in terms of band theory, which says energy levels exist together in

regions known as energy bands. In between those bands are regions known as band gaps, where energy levels for electrons don't exist at all. Some materials have larger band gaps than others. Glass has pretty large band gaps, which means it's electrons require much more energy before they can skip

from one energy band to another and back again. Photons of visible light, that is, light with wavelengths from four hundred to seven hundred nanimeters, corresponding to the colors violet, indigo, blue, green, yellow, orange, and red, these photons simply don't have enough energy to cause this skipping in glass. Therefore, photons of visible light travel right through glass instead of being absorbed or reflected,

thus making glass transparent. At wavelengths smaller than visible light, photons begin to have enough energy to move the electrons in glass from one energy band to another. For example, ultraviolet or UV light, which has a wavelength ranging from ten to four hundred animeters, cannot pass through most sodo lime glass, such as the glass and a window pane.

This makes a window, including the window in our hypothetical house, as opaque to UV light as wood is to visible light, which is pretty excellent considering the damage that EV light can do to where eyes and skin. Pure silica glass would let UV light through. There are other types of transparent glass too, like lead glass, which is sometimes called lead crystal and is used in decorative pieces because it has such high brilliance, especially when cut with lots of facets.

There's also borsilicate glass, which is used for kitchen and lab equipment because it's much better at withstanding temperature changes than lead or soda lime glasses. It actually took humans thousands of years to work out how to make glass clear. Ancient crafts people in Mesopotamia in Egypt some four thousand years ago discovered sodoaline glass and cast it to make

objects like beads and hollow containers. However, natural impurities and the raw materials for glass will cause it to turn colors. For example, iron oxides will create a blue to green to yellow to brownish tint. By around the first century bceeb artisans in Jerusalem invented the technique of blowing glass very thinly so that it appeared transparent mostly, but it wasn't until the fourteen hundreds that Venetian glass makers figured

out how to make glass colorless. They achieved this by carefully controlling the purity of their raw materials and by adding small amounts of other materials to counteract the tints caused by iron oxides like antimony, potash, or manganese oxides. This glass was considered super fancy and also led to the development of technologies like magnifying lenses. All along this timeline and up through today, people working with glass have

developed different techniques to color glass on purpose. Modernly, that's often by adding metals or metal oxides alike copper for red glass or cobalt for blue. When they do, the glass will start reflecting those particular wavelengths of light. Today's episode is based on the article what makes glass Transparent on how Stuffworks dot Com, written by William Harris. Rain Stuff is production of by Heart Radio in partnership with how Stuffworks dot Com, and it's produced by Tyler Klang.

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